Angiogenesis is a process of new vessel formation from pre existing capillaries or venules. It has been reported that this process is impaired in aged tissues. The changes in angiogenesis that occur with aging have been documented at the molecular, cellular, and physiological levels of regulation. The steroid hormones are among the factors that have been shown to have influence on the neovascularization process. Estrogen is one of the steroid hormones which has been suggested to promote new vessel formation. However, regulatory mechanisms underlying the action of estrogen on endothelial cell function and angiogenesis are not fully understood. The action of estrogen is mediated through the estrogen receptor ? (ER??) and/or estrogen receptor ? (ER??). In our study we investigated the role of estrogen receptors in the process of new vessel formation using female ER?? and ER?? knockout mice. Our preliminary data have demonstrated an impairment of skin microvasculature in both ER?? -/- and ER?? -/- mice. Moreover, mRNA levels of several genes involved in the regulation of vessels stabilization such as PDGFB, VE-cadherin, N-cadherin were downregulated in the endothelial cells isolated from ER ? -/- mice. Furthermore, we observed increased collagen synthesis in the ER ?-/- mice, while a decrease was observed in ER?? -/- mice. Interestingly, two proteoglycans, Lumican and Decorin were increased in the skin of ER?? -/- mice, while their decrease was observed in the skin of ER?? -/- mice. These results may indicate an opposite regulation of these ECM components by estrogen receptors suggesting a differential influence on neovascularization process. We hypothesize that ER?? and/or ER?? play an essential role in regulating gene programs mediating vessel homeostasis during chronological aging. To address our hypothesis, we have proposed three aims. In specific aim 1, we will characterize the skin microvasculature in ER?? -/-, ER?? -/- and 12ERKO mice. Since the extra-cellular matrix plays an important role in the vascular network, we will analyze the matrix related genes in mice in vivo in specific aim 2.These analyses will allow us to determine the effects of ER?? and ER?? on ECM production in these animals. In specific aim 3, we will determine the mechanism of ER?? and ER?? on the interaction between endothelial cells and fibroblasts in vitro, using 3D co-culture system. Based on our preliminary in vivo array data we found a significant decrease of adhesion and junction molecules responsible for the cell-cell contact and cell-cell interaction, which may be modulated by ER?? and/or ER??. These studies will allow us a better understanding of the role of estrogen receptors in the regulation of microvasculature during chronological aging.